The present invention relates to the field of multispool and multiflow turbojet engines, particularly twin spool bypass turbojets. It is aimed at mounting an accessory and at the mechanical transmission between this accessory and a shaft of the engine.
To drive the accessories mounted on the engine, such as electric generators, oil pumps or fuel pumps, which are necessary to the operation of the engine or that of the aircraft on which it is mounted, the required power is generally taken off the main shaft. A twin spool turbojet engine has two coaxial shafts, one of them, known as the low-pressure or LP shaft, connecting the low-pressure compressor to the low-pressure turbine and together forming the LP spool and the other, known as the high-pressure or HP shaft, connecting the high-pressure compressor to the high-pressure turbine and together forming the HP spool. In the case of such an engine, transmission of power to the accessories is generally provided by a radial shaft, housed in an arm of the intermediate casing, one end of which has a bevel gear collaborating with a gear secured to the high-pressure spool. The other end is mechanically connected to a box containing a number of gears and forming a support for the accessories while at the same time providing them with drive. When the engine is a bypass engine, the radial transmission shaft passes through both the primary flow and the bypass flow streams, respectively, because the accessory gear box, also known by its English-language acronym AGB, is mounted on the casing of the fan that generates the bypass flow.
Because of evolutions in engine design and engine operating conditions, proposals have been made to mount additional pieces of equipment on the primary flow casing to engage with the radial accessory driveshaft between the two ends thereof. Such pieces of equipment may, for example, consist of a motor for driving the radial transmission shaft, to supplement the drive from the HP spool. Such supplementing is actually useful when the engine layout means that a great deal of power is demanded when this engine is running at idle, in order to drive certain accessories such as electric generators. At idle speed, the HP spool may be unable satisfactorily to supply all the power required in such a design.